Three-dimensional articles produced from conventional hybrid compositions (a composition with both cationically curable and free radically curable compounds) by rapid prototyping techniques (e.g. stereolithography) tend to have low clarity. In particular, said articles may be hazy and/or be highly colored. This haziness is likely due, in larger part, to poor miscibility of the cationically curable and free radically curable compounds, which is accentuated by their different rates of reaction. It is well-known that cationically curable compounds cure slower than free radically curable compounds. In addition, conventional compositions containing certain aromatic acrylates or vinyl ethers, or high concentrations of photoacids, tend to produce colored articles. US 2002/0106584 A1 describes compositions to prepare polypropylene-like articles. Examples show compositions typically containing epoxy compounds, from 18 to 25% by weight of acrylate compounds and hydroxy polycarbonate diol or polytetrahydrofuran polyol. US 2003/0059708 describes photocurable resin compositions containing preformed copolymer and elastomeric particles to form 3D objects free from residual bubbles and having high impact strength.
WO 01/59524 and corresponding US 2001/0046642 describe a liquid, radiation-curable composition which is free from radical photoinitiator. The exemplified compositions all contain polyester polyols.
WO 97/42549 and corresponding U.S. Pat. No. 6,251,557 describe a photosensitive resin composition containing an epoxy resin having aromatic groups and being solid at room temperature, especially a bisphenol A epoxy resin, and an other, liquid epoxy resin, to form 3D articles exhibiting good mechanical properties upon cure. The compositions further contain polyester polyols as caprolactone triol.
EP 0831127 describes stereolithographic resin composition containing thermoplastic polymer compound which is said to mix within the resin composition at room temperature.
WO 2004/001507 is focused on curable compositions containing a benzophenone initiator. The compositions further contain Uvacure 1500 (epoxy compound), SR 399 (monohydroxy dipentaerythritol pentaacrylate), triarylsulfonium hexafluoroantimonate salt.
EP 0837366 A1 describes compositions containing cyclohexene oxide epoxy compound, hydrogenated bisphenol A diglycidyl ether, trimethylolpropane triacrylate, hydroxycyclohexyl phenyl ketone, propylene oxide adduct glycerin.
EP 1437624 A1 describes stereolithographic resins containing selected oxetane compounds. All compositions further contain dipentaerythritol-monohydroxy pentaacrylate SR 399 from Sartomer.
Today, there is a growing need for rapid prototyping materials and processes that produce three-dimensional articles with high clarity as well as improved mechanical properties. For example, manufacturers of end products having a visual aspect, such as computer monitors, portable telephones and televisions, find high clarity resins especially useful.
Early attempts to meet this need focused on using starting materials that contain little or no color. However, this approach proved unsatisfactory because color comes not only from the starting materials but also from the photocuring process itself. Other attempts to improve clarity have relied on removing polyols from the photocurable composition, yet this approach is unsatisfactory because such compositions tend to produce brittle parts.
A continueing need and challenge is to obtain highly transparent cured objects which moreover have very good mechanical properties of tensile strength, modulus, elongation etc.
The type of transparency required relates to both non-haze and no visible colour: ie non-coloured and optically clear objects are sought. To achieve these optical properties, components in a resin mix have to be chosen to be totally compatible and not contain groups [eg aromatics] which can lead to yellowing.
Hybrid resin compositions made up of epoxy, acrylic and ‘tougheners’, eg polyols, all containing certain amounts of hydroxy content are well established in the stereolithography field as providing a good balance of the required mechanical properties. These tend though to ‘yellow’ or become opague with time.
To achieve stable transparency, we need to find suitable component mixes which start off as completely miscible and with no yellow colouration, and which, after the stereolithography or other curing process to consolidate the resin, result in cured objects which also are highly clear and non-coloured.
The present invention relates to a photocurable composition that is especially useful for producing three-dimensional articles with high clarity and improved mechanical properties and thus improved end uses.